The present invention relates to voltage generator circuits, particularly to ramp generator circuits, and more particularly to configurations for allowing a wider variable range of the slope of a ramp wave and to configurations for higher-speed operations in image sensors which perform simultaneous parallel analog-to-digital (A/D) conversion.
Conventionally, an A/D conversion technology using a ramp wave whose waveform changes in a stepwise manner can convert simultaneously, and in parallel, a multitude of analog signals into digital signals. Therefore, attention has been given to such technology as a way of achieving higher-speed operations in a system, such as an image sensor, in which a multitude of pixel signals need to be converted into digital signals.
In this technology, A/D conversion is performed by comparing analog signals provided from individual pixels to a plurality of comparators with a ramp wave, as a reference signal, commonly provided to the respective comparators. In order to support analog signals from pixels having brightness ranging from low brightness to high brightness, a ramp generator circuit having a broad variable slope range is required.
Conventionally, technologies such as those described in Patent Documents 1 and 2 have been known, which change the slope of a ramp wave in a resistor-ladder ramp generator circuit by changing a voltage V applied across the both ends of the resistor ladder circuit as shown in FIG. 19, or by changing a current I flowing through the resistor ladder circuit as shown in FIG. 20.
PATENT DOCUMENT 1: Japanese Patent Publication No. 2008-299716
PATENT DOCUMENT 2: U.S. Pat. No. 6,956,413